1. Field of the Invention
This invention relates to the solvent extraction of a diolefin from a mixture of hydrocarbons. More particularly, this invention relates to the extractive distillation of butadiene or isoprene from a mixture of hydrocarbons.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
An olefin production plant is generally composed of a cracking unit and a hydrocarbons unit.
In the cracking unit a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated.
This mixture, after preheating, is subjected to hydrocarbon thermal cracking using elevated temperatures (1,450 to 1,550 degrees Fahrenheit or F.) in a pyrolysis furnace (steam cracker or cracker). This thermal cracking is carried out without the aid of any catalyst.
The cracked product effluent of the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, or C1 to C35 inclusive, both saturated and unsaturated). This product contains aliphatics (alkanes and alkenes), alicyclics (cyclanes, cyclenes, and cyclodienes), aromatics, and molecular hydrogen (hydrogen).
This furnace product is then subjected to further processing in the cracking unit to produce, as products of the olefin plant, various, separate and individual product streams such as hydrogen, ethylene, propylene, fuel oil, and pyrolysis gasoline. After the separation of these individual streams, the remaining cracked product contains essentially C4 hydrocarbons and heavier. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a crude C5 and heavier stream is removed as a bottoms product and further processed in a depentanizer.
The crude C4 stream can contain varying amounts of n-butane, isobutane, 1-butene, 2-butenes (both cis and trans isomers), isobutylene, acetylenes, and diolefins such as butadiene (1,2-, 1,3-., cis, and trans isomers), vinyl acetylene, and ethyl acetylene, all of which are known to boil within a narrow range, U.S. Pat. No. 3,436,438. Further, some of these compounds can form an azeotrope. Crude C4's are, therefore, known to be difficult to separate by simple thermal distillation.
The crude C5 stream can contain isoprene, pentanes, pentenes, hexanes, hexenes, and aromatics such as benzene, toluene, and xylenes, and can also be difficult to separate by simple thermal distillation.
The crude C4 and C5 streams are normally further processed in the hydrocarbons unit for the separation of other individual product streams such as butenes, butadiene, isoprene, benzene, toluene, and the like.
The crude C4 stream, after removal of acetylenes, normally goes to a butadiene extraction unit for, among other things, the separation and recovery of butadiene therefrom.
The crude C5 stream is normally sent to an isoprene extraction unit for, among other things, the separation and recovery of isoprene therefrom.
A dominating commercial process for separating butadiene from C4's or isoprene from C5's is known technically as “fractional extraction” but is more commonly referred to as “solvent extraction” or “extractive distillation.” However it is termed, this diolefin separation process employs the technique of altering the relative volatilities of various of the compounds to make easier the separation of those compounds by thermal fractional distillation. More specifically, this process employs an aprotic polar compound (solvent) that has a high complexing affinity toward the more polarizable butadiene or isoprene molecules than, and to the substantial exclusion of, the other olefins in the crude C4 or C5 streams. Known solvents used in this process include acetonitrile, dimethylformamide, furfural, N-methyl-2-pyrrolidone, acetone, dimethylacetamide, and the like. This process and the solvents used in it are well known, U.S. Pat. Nos. 2,993,841 and 4,134,795.
The primary equipment employed in the extractive distillation of butadiene or isoprene from a crude stream is an extractive distillation tower or series of towers followed by a diolefin thermal stripping tower and a separate solvent thermal stripping tower. As will be described in greater detail hereinafter, certain equipment that is ancillary to these towers has, from time to time, been plagued with premature and severe corrosion damage. The corrosion rates experienced have been on the order of about 500 mils per year when typical corrosion rates would have been 1 to 2 mils per year.
Until the advent of this invention, the source of this periodic corrosion problem was unknown, and the corrosion itself was difficult to control because the source of the problem was unknown and the corrosion rate was drastic. Pursuant to this invention, the root cause of this corrosion problem has been discovered and removed in an efficient and cost effective manner.